Bacterial abundance and biomass in response to organism-generated habitat heterogeneity in deep-sea sediments

Deep-sea sediments beneath double-ridged crawling trails and around two sessile epibenthic species, the sea anemone Bathyphellia margaritacea Danielssen, 1890 and the demosponge Thenea abyssorum Koltun, 1959, were investigated to test whether they support a bacterial colonisation significantly higher than that of nearby control sediments. The centimetre-scale features and the control sites with a virtually undisturbed sediment surface were sampled in two basin-like depressions in Fram Strait (Molloy Deep, 5552 m; Hayes Deep, 3668 m), and on the eastern Greenland continental rise (3139 m) using a Remotely Operated Vehicle. The uppermost 5 cm-thick layer of the sediment around the two species and beneath the trails contained a significantly higher total bacterial biomass (TBB) than the control sediments. The higher TBB values resulted either from higher total numbers of sedimentary bacteria (TBN), from a higher mean biomass per cell (MBC), or both. Sediments beneath the crawling trails were inhabited by significantly more bacteria than the control sediment. The mean cell biomass of the bacteria did not differ. The TBN around the sea anemone B. margaritacea and in control samples were almost in the same range; the MBC around the anemones, however, was significantly higher than in the control sediments. The higher TBB in the sediment around the sponge T. abyssorum resulted from higher values of both TBN and MBC. Our results confirmed that small epibenthic species and crawling trails sufficiently modify the surrounding sediment-seawater interface to affect the abundance and biomass of sediment-associated biota. The biogenic structures locally increase bacterial colonisation at the sediment-water interface and thus potentially increase habitat heterogeneity for the smallest benthic organisms.